This invention relates to an immersion lithography system and more particularly to methods, as well as systems, for cleaning up the optical element that contacts and absorbs water in the process of immersion lithography.
Immersion lithography systems, such as disclosed in WO99/49504, which is herein incorporated by reference for describing the general background of the technology as well as some general considerations related thereto, are adapted to supply a liquid into the space between a workpiece such as a wafer and the last-stage optical element of an optical system for projecting the image of a reticle onto the workpiece. The liquid thus supplied improves the performance of the optical system and the quality of the exposure.
The liquid to be supplied may be water for light with wavelength of 193 nm although different liquids may be necessary for light with other wavelengths. Because the last-stage optical element of the optical system is exposed to the liquid, there is a possibility that some of the liquid may be absorbed. This possibility is particularly high if the last-stage optical element of the optical system is a lens because calcium fluoride is a common lens material for lithography systems while it is a hygroscopic material that is capable of absorbing water from the surrounding environment.
The absorbed water may cause several problems. First, it may degrade the image projected by the lens by changing the refractive properties of the lens or by causing the lens to swell to thereby change the geometry of the lens. Second, it may cause long-term degradation of the lens due to chemical effects.
Conventional air-immersion exposure lithography systems require the optical elements to be made detachable for maintenance work such as when they are cleaned. It is a cumbersome and time-consuming operation, however, to remove an optical element and to reset it after it is cleaned or to exchange an optical element for a new one.
It is therefore an object of this invention to provide systems and methods for periodically removing the water from the lens such that the amount of absorbed water will not reach a critical level and the degradation of the image and the long-term damage to the lens can be prevented.
It is another object of the invention to provide systems and methods for making the maintenance of the optical element of an immersion lithography apparatus easier and thereby improve the useful lifetime of the optical element.